HBO renegotiated the main stars' contracts using a tiered system that now factors in these potential upcoming episodes. The principle actors were only signed through Game of Thrones Season 6. In exchange for these Season 7 options, HBO has given everyone a substantial raise, which makes the entire cast amongst the highest paid on cable TV.
In April of this year, Game of Thrones was renewed for both Season 5 and Season 6. While this impending seventh season hasn't been green lit, it is considered highly likely given that the HBO fantasy drama is a ratings smash and an awards favorite.
Because the series has such a large and diverse cast, it had to employ this unique 'tier' system. In tier 'A' are stars Kit Harington (Jon Snow), Peter Dinklage (Tyrion Lannister), Lena Headey (Cersei Lannister), Emilia Clarke (Daenerys Targaryen) and Nikolaj Coster-Waldau (Jaime Lannister). They are the highest paid on the series, and they all renegotiated their deals in tandem.
In tier 'B' are co-stars Natalie Dormer (Margaery Tyrell), Sophie Turner (Sansa Stark), Maisie Williams (Arya Stark) and others. While their pay is lower than tier 'A', they all received substantial raises. A lower tier consisting of regular, or recurring actors will receive a much smaller pay increase.
Game of Thrones is currently the biggest show on HBO, drawing in 7.1 million total viewers during Season 4. Across various other platforms, the show pulled in 18.6 million viewers. Its viewership overseas is also said to be incredibly strong. The series led the Emmy nominations with 19 total mentions, including best drama, supporting actor and actress for Dinklage and Headey, respectively; writing and drama series.
Just because the actors listed above have signed on through Season 7 doesn't mean they will all last that long. The series is notorious for killing off its main characters, and any of the actors could be handed their walking papers at any time according to their contracts. George R.R. Martin is still finishing the book series upon which the show is based. Any single one of the actors could potentially lose their job, along with their new paycheck, at any moment without much notice, possibly before a seventh season even debuts.